The present invention relates to a system utilizing fuel stacks.
Such a system is known from xe2x80x98Molten Carbonate Fuel Cell Networks: Principles, Analysis and Performancexe2x80x99 of J. G. Wimer c.s., 28th IECEC, Atlanta, 1993. A number of MCFC fuel cell stacks is used because the practical size of a fuel cell stack is limited.
A first reason for such a limit is that the fuel cell stack must be small enough to be transported with ease to and from the site. Secondly it may be difficult to ensure an even gas flow distribution in a large stack. For these reasons molten carbonate fuel cell stacks may be limited to about 200 kW.
In the system according to Wimer both the cathodes and anodes of subsequent stacks are connected in series. For the anode, this results in increased pressures in the anode passages of the first (upstream) stack so the stack components have to be designed to withstand relatively high pressures.
EP-0 422 352 A2 describes a further multi-stack system. In this system CO2 is removed from the oxidant outlet stream of a stack and returned into the oxidant inlet of the same stack. Two-stack systems are described in which the second stack acts as a CO2 separation device for the oxidant outlet stream of the first stack. An embodiment, described in this European patent, includes series connection of the oxidant flows without cooling between stacks. Therefore the oxidant gas inlet temperature of the several stacks will be different. Because of that a high oxidant gas flow is required to cool the stacks resulting in low performance.
If a system according to the European patent application 0 442 352 A2 would be extended to systems with more than two stacks, anode outlet gas from all the stacks except the most downstream stack would be circulated through the cathode inlet of the same stack. This would mean that a burner had to be placed upstream of the cathode inlet of all stacks except one. This means that the system according to the European patent application 0 442 352 A2 is not practical to realize with more than two stacks.
The invention aims to provide a system which allows the use of stacks of a single design while retaining the advantages of series connection of the oxidant streams and giving low per pass fuel utilisations. Furthermore the invention aims to provide a system in which the highest absolute pressure in the anode part of the stacks can be lowered whilst the efficiency is increased. Besides, a minimum number of heat exchangers is needed and the temperatures are controlled by relative cheap low temperature valves.
Series connection of the oxidant streams gives a higher cathode gas flow rate per fuel cell stack than an equivalent parallel connected system. With series connection the cathode flow passes through each fuel cell stack and performs stack cooling on each pass. Consequently the cathode gas performs more stack cooling when flows are connected in series with cooling between the stacks than when they are connected in parallel with the same inlet and outlet temperature. This is advantageous as it allows either a reduction in stack outlet temperature or an increase in fuel utilisation compared to the equivalent parallel connected system. Reduction in outlet temperatures extends the life of the fuel stack. Increase in fuel utilisation gives a higher electrical efficiency.
The parallel connection of the anode streams ensures that all the anode channels are at essentially the same pressure. Furthermore if an anode recycle blower would be present it needs to produce a pressure sufficient to overcome the pressure lost in only a single fuel cell stack.
The present invention gives a low single pass utilisation per stack for both the anode and cathode flow at a high overall fuel utilisation. Owing to the low single pass utilisation local depletion, a non-uniform flow distribution within the stack will not be encountered and this gives a considerable advantage over the prior art. Besides anode recycle reduces the increase of the anode flow in the cells of the stack, which is favourable for the flow distribution. Also no steam injection is needed for the reform reaction and the prevention of carbon disposition.
It has to be understood that this system can be used for any type of fuel cell operating at relatively high temperature, such as a SOFC or MCFC.
Series connection has been proposed previously. However, systems known in the prior art do not allow internal reforming stacks to operate with the same oxidant gas inlet temperature, fuel gas inlet temperature, stack outlet temperature and very similar oxidant gas flow rates and fuel gas flow rates. This is possible with the arrangement according to the present invention. The same stack design can therefore be used for all the stacks and this reduces manufacturing cost.
In the system according to WIMER a combustor is provided being fed by oxidant, which is branched from the most downstream cathode exhaust and the fuel gas is branched from the most downstream anode exhaust. This mixture is transferred after its combustion and after heat exchange to lower its temperature to the cathode feed side of the most upstream fuel cell stack.
To practice this solution there are several drawbacks. First of all the percentage oxygen in the oxidant entered into the combustor is relatively low and this is more particularly true if the oxidant is air. Because of that combustion will be difficult if the percentage of combustible gas in the anode exhaust gas is relatively low. This means that special measures may have to be taken to ensure complete combustion of any fuel gas from the anode.
Secondly a high temperature pumping means such as a blower is needed to produce the flow of cathode exhaust gas to the combustor.
According to a further embodiment of the invention this drawback is obviated in that the air is connected with said pumping means, said pumping means being connected on the other hand with the feed of the exhaust gases of the anode outlets to the combustion device and the feed of oxidant gas to the cathode inlets between the several fuel cell stacks. The heat exchanger between the combustor and the first cathode inlet as used in the system according to WIMER can be omitted.
This embodiment of the invention is based on the idea to use fresh air in the combustor and to position the compressor upstream from this combustor so that it is not subjected to the high temperature and/or corrosive gasses from the combustor.